Some wavelength-division-multiplexing-passive-optical-networks require precise wavelength alignment between the wavelengths of the signal from a transmitter in a central office to a device in a remote site distributing that signal to a subscriber. In a passive-optical-network, a remote node containing the signal-distributing device is typically located outdoors without any electrical power supply. The transmission band of wavelengths of the outdoor signal-distributing device can change according to the variation of the external temperature. Misalignment of the wavelength between the transmitted signal and the operating wavelength of the device distributing the signal introduces extra insertion loss in the signal.
A possible way to minimize the misalignment can be to use a narrow-linewidth distributed feedback laser diode (DFB LD) that essentially always falls within the shifting bandwidths of the multiplexers as an optical transmitter to satisfy the wavelength alignment condition. However, this arrangement may not be an economic solution because of the high price of each DFB LD.
Further, in systems having two or more wavelength division multiplexers located in different areas, misalignment of the transmission band of wavelengths between the two or more wavelength division multiplexers may occur due to the temperature variations at these different areas. The operating transmission band of wavelengths of these devices can vary depending on the temperature of the device.
Complex channel selection and temperature control circuits could be employed to compensate for the large insertion loss in optical signals passing through optical multiplexer/demultiplexers located in different locations. However, the complexity of the channel selection circuit has the disadvantage that the complexity of the circuit becomes greater and greater as the number of input ports of the circuit increases. Thus, the more channels being distributed by a multiplexer/demultiplexer, then the more complex and expensive the channel selection and temperature control circuit becomes.